This invention relates to a method of rapidly heating a NMR sample and an apparatus for using such a method. More particularly, this invention relates to a method and an apparatus for heating an NMR sample to room temperature or above from an initial temperature which is sufficiently low for polarizing the sample to a desired degree within a time comparable to its thermal relaxation time.
There is a class of experiments in which it is desirable to rapidly heat a NMR sample from a very low temperature to room temperature or above. The degree of polarization normally achieved by a NMR sample is inversely proportional to the absolute temperature and proportional to the magnetic field strength. For example, the thermal polarization of a sample at a temperature of 3xc2x0 K is 100 times greater than the polarization produced near room temperature, i.e. about 300xc2x0 K. If the sample can be quickly warmed from 3xc2x0 K to 300xc2x0 K within a time period comparable to its thermal relaxation time, a high resolution NMR spectrum can be achieved with greatly enhanced sensitivity. To achieve the same sensitivity at room temperature would require a time period roughly 104 times longer.
The solids effect can be achieved in solids containing electron radicals, i.e., unpaired electrons that are coupled to nearby nuclei. (See, for example, R. A. Wind et al., xe2x80x9cApplications of Dynamic Polarization in 13C NMR in Solidsxe2x80x9d, Progress in NMR Spectroscopy, Vol. 17, pp 33-67, 1985. In particular, see Sec. 2.3, The Solid State Effect). RF or microwave irradiation of the solid near the frequencies xcexde+xcexdn or xcexdexe2x88x92xcexdn causes simultaneous electron and nuclear spin flips, where xcexde is the electron Larmor frequency and xcexdn is the nuclear Larmor frequency. The population redistribution results in an enhanced nuclear polarization. The enhancement can be substantial, approaching the ratio of xcex3e/xcex3n. For proton nuclei this ratio is approximately 650 and 3,400 for 13C.
By combining the two effects, i.e., achieving a high thermal polarizationation at low temperatures, using the solid effect to further enhance the nuclear polarization while at the low temperature, one can achieve a substantial nuclear polarization. By quickly warming the sample much of this polarization can be maintained even after the sample melts, enabling one to achieve very high sensitivity NMR experiments upon liquid samples. The nuclear relaxation time T1 is a measure of how long this excess nuclear polarization time lasts. At low temperatures, near the initial temperature of the sample when it was polarized, the relaxation time is typically very long, several minutes to hours. As the sample melts, the relaxation time becomes much shorter, perhaps in the range of a second or less to tens of seconds. After the sample has been polarized to achieve an initial polarization P0, the heating process is started. As the sample is heated it begins to lose some of its polarization. After a time xcfx84, the remaining excess polarization, P, is given by the approximate formula:
P=P0 expxe2x88x92∫0xcfx84dt/T1(t)}
Using standard NMR procedures the relaxation time T1 of the sample can be measured for various temperatures. The sample temperature can also be measured for various pre-selected heating times. Thus for a given experimental relaxation time, T1, can be expressed as a function of time, T1=T1(t), enabling one to integrate the expression above and obtain an estimate of the excess polarization. Typically the heating rate should be sufficiently rapid that ratio of excess polarization P to the initial polarization P0 be down not more that a factor of 20, therefore P/P0 greater than 0.05.
In a typical experiment, the analyte (material to be analyzed by NMR) is dissolved in a mixture of 40:60 solution of water/glycerol with the free radical 4-amino TEMPO as the source of electron polarization. (See C. T. Farrar et al., xe2x80x9cHigh-Frequency Dynamic Nuclear Polarization in the Nuclear Rotating Framexe2x80x9d, J. Magn. Resonance, Vol. 144, pp 134-141, 2000). As pointed out in this reference, other dynamic nuclear polarization techniques may also be used to polarize samples at low temperature.
The microwave sources used to produce the microwave transitions range from low cost solid state oscillators such as an impact diode or Gunn oscillators to high power gyrotron oscillator tubes such as manufactured by CPI.
Golman and Ardenkjaer-Larsen have suggested the method of quickly warming the sample by adding a hot solvent. This method, however, has the effect of diluting the sample further and does not permit repeated experiments with the same sample unless it is purified between successive spectral runs.
It is therefore an object of this invention to provide a method and an apparatus for using the solids effect to polarize a sample while it is very cold and then rapidly heating it to a liquid state near room temperature within a time sufficiently short to prevent substantial loss of polarization during the heating process so as to enable narrow line liquids NMR data to be taken while a high degree of polarization remains.
It is another object of this invention to provide such method and apparatus which will not dilute the sample being heated such that experiments can be repeated many times on the same sample.
In one embodiment of this invention, the sample is rapidly heated by radiation. This is achieved by rapidly moving the pre-polarized sample to the center of a furnace tube. Analysis shows that a 5 mm sample tube of water (4.2 mm ID) could be heated from 1xc2x0 K to 303xc2x0 K (30xc2x0 C.) in about 14 seconds for a furnace tube operating at 1000xc2x0 K (1273xc2x0 C.). After a proper amount of heat is absorbed, the sample could be dropped into the NMR probe for analysis.
An alternative heating method involved using a sample tube with embedded heater wires that produce heat by passing a current through them. The heater section of the sample tube could be located at one end of the sample tube. With the sample initially at the upper end of the sample tube so that after the sample is heated and melted, the sample would collect at the lower end of the tube free of the heater wires. The melted sample is now placed in the NMR spectrometer for analysis.
The processes described above can be carried out automatically after the sample containing the free radical and solvent is once loaded into the sample tube and sealed.